1. Field of the Invention
This invention relates to the field of optical scanning, and, more specifically, to optical scanning methods and systems for scanning with controllable beam deflectors.
2. Background Art
An Acousto-Optic Beam Deflector (AOBD) generates a scanned laser beam as shown in FIG. 1a. By varying the acoustic frequency of the AOBD, the grating period and hence deflection angle is varied. However, various aberrations can be introduced to the optical system from the AOBD. As is well known in the art of acousto-optic scanning, a cylindrical lens effect results when the grating period varies along the long axis of the oblong-shaped scan aperture during scanning as shown in FIG. 2a. As the AOBD is scanned at higher rates, variation of the grating period across the aperture increases and the lensing effect increases. Other well-known effects include the lateral chromatic dispersion of the AOBD grating which varies with scan angle, but is independent of scan rate.
The so-called off-axis AOBD exhibits an acoustic walk-off angle such that the deflection is imparted at a steeply tilted active scan pupil lying within the AO crystal. This walk-off angle introduces additional aberrations to the optical system. The tilted scan pupil creates a significant variable beam width in the scan axis as shown in FIG. 1b. In the absence of cylindrical lensing, for example when the device is used to point at stationary angles, the width variation would result in an elliptical beam shape error and therefore elliptical focused spot shape variation with scan angle.
However, during dynamic scanning, especially at preferred high scan rates, especially when there is significant cylindrical lensing, the lens effect aberration is compounded with the variable beam width effect. As the beam width varies through a scan, the wavefront curvature resulting from the cylindrical lensing also varies so that the effective focal length corresponding to the wavefront curvature of the cylindrical lens effect varies. This focal length variation results in a tilted in-scan focus as the beam is scanned as shown in FIG. 2b. The focus tilt is itself non-linear, so that field curvature also results. In addition to the cylindrical lens effect, focus tilt and field curvature, coma is imparted from the cylindrical focusing power located at the tilted scan pupil. By far the most significant aberration is focus tilt
The various optical aberrations described with the use of the off-axis AOBD can result in larger imaged spots, out-of-round spots, and result in limits to the performance of the optical scanning system. It is therefore desirable to sufficiently mitigate the optical aberrations introduced by the off-axis AOBD to be able to achieve high speed scanning while limiting degradation of optical performance.
Sandstom in U.S. Pat. No. 5,517,349 discloses use of a prism or off-axis lens to correct for aberrations introduced by the off-axis AOBD. This method requires a deflection element after scanning the beam. The deflection element is used to reverse the beam width variation effect and thereby correct aberrations.
Engelhardt in U.S. Pat. No. 6,754,000 discloses use of an adaptive optic element to introduce wavefront correction before scanning. The pre-corrected wavefront interacts with the AOBD and phase errors are cancelled. The adaptive optic is a dynamic device, so varying correction can be applied across a scan to correct for focus tilt.
Nikoonahad in U.S. Pat. No. 5,663,747 discloses varying the chirp dispersion rate of an AOBD from a nominally corrected dispersion rate. In this way the scanned spot size at the surface of a patterned wafer is varied from a diffraction limited size by defocusing.
The following U.S. patents are related to the present invention: U.S. Pat. Nos. 2,354,614; 3,972,584; 4,090,775; 4,213,690; 4,672,458; 4,946,234; 4,978,860; 5,517,349; 5,557,446; 5,663,747; 5,754,328; 5,828,481; 5,900,993; 6,719,430; 6,754,000; and 6,775,051.
The following reference is also related to the present invention: Leroy D. Dickson, “Optical Considerations for an Acoustooptic Deflector,” APPLIED OPTICS, Vol. 11, No. 10, October 1972, pp. 2196-2202.